This invention relates to an engine control system responsive to engine operating parameters for determining a target value of setting of a controlled variable to obtain an optimum value of a condition for efffecting a predetermined function of an internal combustion engine and, more particularly, to such an engine control system having a feedback control circuit for correcting the target value for a deviation between the optimum and actual values.
There has been an increasing need for employing microcomputers in engine control systems for more accurately controlling one or more functions of an internal combustion engine. Such a microcomputer calculates target values of settings of controlled variables such as fuel-injection timing, EGR-valve position, ignition-system spark-timing, and the like and corrects each of the calculated target value for a correction factor corresponding to a deviation between the calculated target value and sensed actual value for the controlled variable. The correction factors are stored in a computer memory such as a random access memory (RAM).
For example, with respect to the controlled-variable EGR-valve position, which determines the amount of engine exhaust gases recirculated to the combustion chambers of the engine, a target value is calculated to obtain an optimum EGR rate in relation to engine speed and engine load. In order to correct the target value for changes with the passage of time due to carbon accumulated in the EGR system, a correction factor is obtained in accordance with a deviation between the target EGR rate value and an actual EGR rate value sensed by a suitable sensor. It is necessary to store the correction factor in the computer memory or use just after the engine, which is stalled once, is restarted. However, the correction factor stored in the computer memory will be erased when the computer memory is separated from a source of power.
It is the normal practice to maintain the data stored in a computer memory by using a circuit for maintaining the computer memory connected to a vehicle battery. However, the vehicle battery is required to be detached during vehicle repair or long parking so that the stored data will be erased. Alternatively, a nonvolatile RAM may be used to store the correction factor. However, this results in an expensive engine control system.
The present invention provides an engine control system having a memory which includes a potentiometer and a reversible motor drivingly connected to the potentiometer to tap off a resistance corresponding to a value to be stored in the memory. Thus, the stored value is maintained after the memory is separated from a source of power.